1. Field of the Invention
The present invention relates to a device and a method for laser marking by which laser beams are irradiated onto a web-like material, such as a photosensitive material or a heat-developing photosensitive material, to be printed, and a marking pattern of characters, marks, or the like is formed.
2. Description of the Related Art
When characters, marks, or the like are marked onto a photosensitive material such as an X-ray film, laser beams are used in some cases. The X-ray film absorbs the energy of the irradiated laser beams to cause dot-like fogging and deformation. In a marking method using the laser beams, a marking pattern of characters or marks based on a dot array is formed by irradiating the laser beams onto the X-ray film while the beams are scanned.
In order to improve the visibility of the marking pattern formed on the X-ray film, the dots are required to be formed with a suitable size.
Then, adequate control of the laser beams is needed in order to form the dots with a suitable size and shape on the X-ray film by scanning the laser beams.
For example, in the Japanese Patent No. 3191201, combinations of energy densities and pulse widths of laser beams have been proposed as marking conditions for a case in which the laser beams are irradiated onto a photosensitive material such as an X-ray film, and dots which are almost circular are formed at a predetermined interval for marking. Specifically, energy densities have been proposed for forming dots with excellent visibility onto the X-ray film when laser beams with pulse widths within a range of 30 μsec to 200 μsec are irradiated.
However, when the X-ray film is carried at high velocity in order to improve the productivity of the film, there is a possibility that deviation of dot positions is caused, or that the dots required for forming characters, marks or the like cannot be formed completely because the radiation time of the laser beams becomes too long under a condition in which the pulse widths are within a range of 30 μsec to 200 μsec.
When, for example, a character of 5×5 dots is printed, using a line of laser beams, a linear velocity V (m/min) corresponding to a pulse width t (μsec) for the radiation time of the laser beams is approximately shown as follows: V=3000/t. However, when the pulse width t is 30 μsec, the X-ray film cannot be carried at a velocity of 100 m/min or more.
Moreover, when an X-ray film with a higher sensitivity is marked while the film is carried at low velocity, it is preferable for preventing quality degradation such as fogging to use laser beams with smaller energy densities. Especially when the pulse widths are 30 μsec or more, a longer radiation time of the laser beams causes a corresponding increase in the total energy amount supplied to the X-ray film by radiation, and not only the surface, but also the inside of the X-ray film is melted. Accordingly, there is a possibility that the visibility of the dots is reduced or that quality degradation such as fogging is caused.
Incidentally, among processing methods using laser beams, there is a method for processing the surface of a material to be processed by which laser beams are irradiated onto the surface of the material to be processed and the surface is melted or the like by the heat of the laser beams for processing.
As one method for using laser beams, there is a marking method by which dot-like processed signs are formed by irradiating the laser beams on the surface of a material to be printed, and characters, marks, and the like are formed by use of a dot array comprising the processed signs.
For example, dot-like fogging and deformation is caused on a photosensitive material such as an X-ray film by absorbing the energy of the laser beams irradiated onto the film. Accordingly, the laser beams are scanned and irradiated onto the photosensitive material such as an X-ray film to form a marking pattern of characters and marks comprising dot arrays.
Furthermore, when a material to be printed is a web-like photosensitive material or the like, and laser beams are irradiated onto the surface of the material to be printed for forming a marking pattern, the laser beams are scanned and irradiated onto the photosensitive material while the photosensitive material is being carried.
For example, Japanese Patent Application Laid-Open (JP-A) Nos. 2001-239378 and 2001-239700 propose winding a photosensitive material onto the peripheral surface of a back-up roller, and irradiating laser beams onto the surface of the photosensitive material wound onto the roller in such a way that the laser beams are focused at a predetermined position on the surface of the photosensitive material.
For example, Japanese Patent No. 3191201 proposes setting the energy density and the radiation time of laser beams at a predetermined value in order to form dots with excellent visibility on a photosensitive material.
When the laser beams are irradiated on an X-ray film during laser marking, heat is generated on irradiated parts by the laser beams. When the heat is not transmitted to a back-up roller and remains in a photosensitive material, defective performance, such as sensitization or desensitization, or quality degradation, such as thermal fogging, is caused on the photosensitive material.
For example, Japanese Patent No. 3202977 proposes a structure in which a flexible wiring board onto which laser beams are irradiated is held by suction on a receiving board to prevent deviation of focal positions by deflection. In the structure, the receiving board is made of a metal plate with a heat transfer coefficient of 8 W/m×K or more in order to secure heat radiation.
However, there is a possibility that quality degradation, such as thermal fogging is caused on a photosensitive material, even if the outer peripheral part of a back-up roller is formed using a material with this degree of the heat transfer coefficient.
Moreover, a problem occurs in that the heat transfer coefficient is reduced by an air layer which forms between a material to be printed and the back-up roller due to entrained air when the web-like material, such as a photosensitive material, to be printed is wound onto the back-up roller.